High solid polyester polyol resin is an important, environment-friendly resin, and is usually prepared by condensation polymerization of diol and/or polyol with diacid monomers. It has been widely used in high solid automotive coatings, furniture coatings, coiling coatings etc. From the viewpoint of environment protection, lower viscosity of polyester polyol resin is preferred, which means lower VOC in its corresponding coatings. Generally, the viscosity of polyester polyol is reduced by increasing the molar ratio of diol(and/or polyol)/diacid. However, high dosage of polyol monomer in condensation polymerization will produce polyester polyol resin with high hydroxy value and thus more crosslinking agent is needed in curable coating compositions. Meanwhile, a polyester polyol resin with a molecular weight that is too low, will negatively impact the properties of coatings such as impact resistance. Thus, to attain a proper balance between the VOC and the properties of coatings, the reduction of molecular weight of polyester polyol resin is often limited for certain monomer composition. In other words, besides the composition of polyester polyol resin, adjusting the molecular weight of polyester polyol resin is also limited to improve the properties of polyester based coatings.
In recent years, accompanying with development of nanotechnology and nanomaterial, inorganic nanoparticles such as silica, alumina, zirconia and so on were incorporated into the coatings to prepare transparent coatings with improved abrasion and scratch resistance, hardness etc. Among them, nanosilica in both fumed silica and colloidal silica state was most widely employed because of its relatively cheap price and better match of refractive index with most of the resins. To achieve the better dispersion and compatibility of nanosilica with coatings or offer the reactivity with the binder in coatings, surface modification of nanosilica particles was usually adopted. In U.S. Pat. No. 5,853,809, colloidal silica was modified with functional silane coupling agent such as carbamate functional silane coupling agent before introducing into clearcoat. While in U.S. Pat. No. 6,803,408, colloidal silica was firstly dispersed in pre-prepared polysiloxane polyol and then mixed with other component of coatings. Besides surface modification, a special dispersing method, namely jet dispersion method through nozzle at high pressure, was also proposed to successfully incorporate hydrophobic fumed silica into acrylic based polyurethane clearcoat, just as described in U.S. Pat. No. 6,020,419. Transparent nanocomposite coatings with improved scratch resistance were all obtained in the above reported patents. But unfortunately, the surface modification of nanosilica will complicate the preparing process of nanocomposite coating and the remained free modifying agents maybe reduce the copolymerization rate (specially for the case that vinyl containing silane coupling agent modified inorganic nanoparticles firstly dispersed in acrylate monomer) and/or deteriorate the properties of the resulted coatings. For jet dispersion method, a specific dispersing apparatus, which is distinctly different from conventional mechanical stirrer or miller used in coating manufacture, has to be set up and thus increase the investment. Thus, development of a simple process for preparation of transparent nanocomposite coating is very necessary.
Colloidal silica particles exist in the form of individual dispersion in the media such as water, alcohol and other organic solvent. Usually, there are large amount of hydroxyl group on its surface. These hydroxyl groups will lead to form strong hydrogen bond interaction between colloidal silica and resins especially those high degree hydroxyl functionalized resins, or between colloidal silica particles themselves. Higher hydrogen bond interaction means higher viscosity of colloidal silica containing resin and coatings. Therefore, colloidal silica particle was ever used to improve the rheological properties of high solid coatings in previous patents such as U.S. Pat. No. 4,526,910. However, the thickening role of colloidal silica is not welcome when colloidal silica is acted as the additive for improving the mechanical properties of high solid coatings because it will strongly decrease the incorporation amount of colloidal silica in coatings. Therefore, shielding the surface hydroxyl group of colloidal silica is needed. Just as described above, surface modified with modifying agent such as functional silane coupling agent was efficient to protect the surface of colloidal silica, but it is complicated.
To simply the process of application of colloidal silica in coatings while maintaining the efficiency of colloidal silica in improving the properties of high solid coatings, a novel process was proposed especially for preparation of high solid polyester polyol/silica nanocomposite resin. The resin is curable with isocyanate or melamine to obtain high solid nanocomposite coatings which can be used as scratch resistant automotive coatings, coil coatings, plastic coatings, metal coatings etc.